Thermally actuated dual magnetic switch with shunt and temperature compensation



Apnl 21, 1964 T. E. LARSEN 3,130,285

THERMALLY ACTUATED DUAL MAGNETIC swI'rcH WITH SHUNT AND TEMPERATURE COMPENSATION Filed March 11. 1960 COOLING APPARATUS o lol' 90 Emi HEATING APPARATUS 1' -5 I zzvmvrox 7 THEODORE E. LARSEN ATTORNEY Uflied a e Patent 3,130,285 THERMALLY ACTUATED DUAL MAGNETIC SWITCH WITH SHUNT AND TEMPERATURE COMPENSATION Theodore E. Larsen, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Mimn, a corporation of Delaware Filed Mar. 11, 1960, Ser. No. 14,428

- 8 Claims. (Cl. 200-138) The present invention is concerned with an improved switch particularly adapted for use in control apparatus which has a minimum space requirement. The switch makes use of two magnets which are moved in parallel planes with one magnet operating between fixed magnetic stops so that an action is converted from one magnet to the other through the repelling force to actuate contacts associated with one of the magnets. I

In small control devices such as thermostats, the use of a sealed tube mercury switch has produced increased reliability over the open contact designs heretofore used. With the increase in cost of the sealed tube mercury switch, a need for an inexpensive and reliable switch which could beused to replace such mercury switches became more apparent. The present invention is concerned with a simple switch which has a first switching magnet. The switching magnet is contained in a sealed chamber and carries at least one contact which engages v a fixed contact when the magnet is moved to a predetermined position. A second drive magnet is mounted outside of the chamber to magnetically couple the two magnets to produce operation of the switch when the drive magnet is moved by some condition responsive device. By reducing the amount of friction and mechanical contact of the various parts, the improved switch operates become apparent upon the study of the following specification and drawings of which:

FIGURE 1 is a front cutaway view of a control apparatus making use of the improved switch mechanismp FIGURE 2 is a front cutaway view of the switch mechanism showing theswitch magnet which is contained in a sealed chamber.

FIGURE 3 is a side view of the switch mechanism of FIGURE 1. I 7

FIGURE 4 is a top view of the switch mechanism as shown in the control apparatus of FIGURE 1.

FIGURE 5 is a top view of the switch mechanism showing another relationship of the two repelling magnets shown in FIGURE 4. I

FIGURE 6 is an enlarged view of the drive magnet and its stop.

Referring to FIGURE 1, a typical control device such as a thermostat contains the improved switch. A base member 10 which has an attached cover 11 supports a switch mechanism 12 and a condition responsive element or bimetal 13. The bimetal is connected to a drive lever 14 of the switch mechanism; so that, upon changes in the temperature surrounding the control device, lever 14 is moved to the left. A gear sector 15 is connected by a linkage to adjust the position of bimetal 13 when a control point adjustment knob 20 is rotated. Knob 20 is mounted to base 10 by a shaft 21. Switch mechanism 12 is connected into an electrical circuit by terminal 26, wire 22, and terminal strip 62 shown in FIGURE 2.

Referring to FIGURE 1, the switch mechanism 12 is shown in detail. A housing 24 forming a sealed chamber 25 is attached to base 10. A pair of adjustable, ferromagnetic stops (steel screws 30 and 31) are attached to the front side of housing 24 by means of projections 32 and 33 which are an integral part of the housing. A drive magnet 34 is positioned in front of housing 24 for movement in a plane parallel to the housing when motion is transmitted through lever 14. Magnet 34 which is oriented to have a north and a south pole axis in a line perpendicular to the plane of the housing is connected to lever 14 by a metal framework 35 surrounding the magnet. Magnet 34 moves between a firstand second position as determined by stops 30 and 31 in a snap acting manner whenever force is applied either to the left or the right upon a change in the temperature of bimetal 13. Stops 31 and 30 can be adjusted to vary the amount of ferromagnetic material in the magnet circuit when magnet 34 is against one or the other of the stops. The movement of magnet 34 is maintained in a plane parallel to housing 24 by a flexible blade 40 between lever 14 and base 10 as shown in FIGURE 3. Blade 40 is connected to a free end of spring blade 41 which isattached to base 10 by screw 42. A second screw 43 is to adjust the relative position of magnet 34 and the base; so that, when screw 43 is moved into base 10 in FIGURE 1, magnet 34 moves closer to housing 24.

Contained inside chamber 25 as shown in FIGURE 2 is a switch magnet 50 which is pivotally supported from a bracket 51. Switch magnet 50 is oriented to have a north and a south pole on a line perpendicular to the plane of the housing. The same poles of magnets 34 and 50 are adjacent so the magnets repel each other. Bracket 51 is attached to housing 24 (by means of eyelets) and electrical connection is made to terminal 26 (by means of rivets passing through base 10). A flexible spring 52 connects bracket 51 and a metal framework 53 surrounding magnet 50. Framework 53 which is made of metal having good electrical conductivity characteristics such as silver forms the movable contacts of the switch mechanism. I

A fixed contact 60 is contained in chamber 25 to be engaged by contact 53 when magnet50 moves to the left as shown in FIGURE 2. A similar contact 61 is located on the right side of chamber 25. Contacts 61 and 60 are flexible blades attached at their upper ends to housing 24 so a certain amount of pressback is available if necessary. The contacts actually move laterally when they engage to perform a wiping action which maintains the contacts clean. Wire 22 is connected to blade or contact 60, and blade 61 is connected to terminal strip 62 (strip 62 and wire 23 terminate at the same point).

Switch mechanism 12 is made up of switch 53-60 and switch 5361 formed when magnet 50 and framework 53 engages blades 60 and 61, respectively. When bimetal 13 moves magnet 34 to the farthermost right-hand position against stop 31 as shown in FIGURE 1, magnet 50 I is repelledand is moved to the left as shown in FIGURE 2. Framework 53 which supports magnet 50 and makes up the movable contact engages contact 60. Magnet 50 can only move a limited amount as determined by the distance between stop 70 and a similar raised portion stop 71 associated with blade 61. The magnets are shown in the mentioned positions in FIGURES 4 and 5.

Upon a change in the output of bimetal 13 sufficient to overcome the force of attraction between magnet 34 and magnetic stop 31, magnet 34 is moved to the left. magnet 34 snaps against stop 30, magnet 50 is again 3 repelled, but in the opposite direction against contact blade 61 and stop 71. Stops 70 and 71 limit the travel of magnet 50 which is important in keeping the differential of the switching mechanism low.

Referring to FIGURE 6, drive magnet 34 with the ferromagnetic framework 35 is shown against stop 31. Magnets such as magnets 34 and 50 might be made of unorientated barium ferrite which has a high temperature coefficient of magnetization or the characteristic of having a large change in flux density with changes in temperature. In most control devices such as temperature control devices, the temperature can change a fairly wide range such as 62 to 90 Fahrenheit. Magnet 34 with a non-ferromagnetic framework 35 would experience a change in attractive force between the magnet and stop 31 as the temperature changed. As the operation of the device depends upon a constant force between the magnet and the stop, a shift in the operation of the control device using the switch mechanism would be experienced. For example, at a low temperature such as 62, the force to pull magnet 50 to the left away from the stop may be a certain amount; however, at a higher temperature such as 85, a smaller force would be necessary to pull the magnet 34 to the left. The amount of ferromagnetic material in the flux path 80 can be changed by adjusting stop 31; however, for a fixed amount of stop, the total flux of magnet 34 will vary as temperature changes.

The framework 35 holding magnet 34 is a ferromagnetic material which has a high temperature coefiicient permeability. The permeability of the material decreases when the temperature increases. Framework 35 actually provides a magnetic shunt circuit; so that, magnetic flux flowing from one pole of magnet 34 around to the other pole of the magnet flows through the magnetic shuntor framework 35. The magnetic flux flowing along path 80 as shown in FIGURE 6 produces the attractive force between the magnet 34 and stop 31. As the temperature increases, the amount of flux developed by magnet 34 decreases, and even if all of the developed flux flows along the path 80, the attractive force-between magnet 34 and stop 31 would decrease as the temperature increased. With the ferromagnetic shunt 35, a certain portion of the flux produced by magnet 34 flows along the path 81; however, the resistance for the flux flow throughpath 81 varies with the temperature of ferromagnetic shunt 35. The resistance varies directly as the temperature; so that, at a low temperature, such as 62, a large amount of flux produced by the magnet 34 flows alongpath 81 throughthe shunt. As the temperature of the magnet and shunt increases, the resistance to flux flow through shunt 35 is increased, and a greater proportion of the flux developed by magnet 34 flows along path 80 to provide the attracting force between magnet 34 and stop 31. Since the total amount of flux developed by magnet 34 at the higher temperature is less and less flux flows through the shunt circuit 81, the net flow of flux along path 80 remains substantially constant. The result of the addition of the ferromagnetic shunt 35 is to maintain a constant attractive force between magnet 34 and stop 31 even though the temperature changes over the range mentioned.

Switch magnet 50 has the similar high temperature coefficient of magnetization as magnet 34. Framework 53 not only holds magnet 50, but the framework forms the movable contacts of the switch and the connection means between the movable contacts and terminal 26. While the framework53 is only shown as a thin section of metal surrounding the magnet, other modifications to increase the ability of the framework to provide a good switch action when it engages either switch contact 60 or switch contact 61 might be made. The use of silver to increase the conductivity and contact life of framework 53 would greatly improve the switch operation. Since the current which is conducted between the wire 22 or terminal strip 62 to terminal 26 when magnet 50 is in the left or right position, respectively, is made to pass through framework 53, the temperature of magnet 50 increases due to the heat developed by the current flow through the switch. As previously mentioned, the temperature coefiicient of magnetization is a reduction in the flux density as temperature increases. When the temperature of magnet 50 increases, the effective force developed by the repelling action of magnets 34' and 50 is reduced. If the distance between magnets 34 and 50 is changed by moving screw 43, the repelling force between the two magnets is changed and the switch operation is affected. The temperature increase on magnet 50 due to the switch heat has the effect of varying the distance between magnets 34 and Sti and thus the repelling force between the two magnets.

Operation Let us assume that the switch mechanism 12 as shown in FIGURE 1 is connected to control heating apparatus 100 and a cooling apparatus 93 by the appropriate Connections. A circuit for the cooling apparatus is tracedas follows: from a source of power 90, a conductor 91, terminal 26, switch 5361, terminal 62, a conductor 96, cooling apparatus 93, a switch 101 and back to the source of power. A circuit for the heating apparatus is traced as follows: source of power 90, conductor 91, terminal 26, switch 53-60, wire 22, an adjustable anticipation heater 94, a terminal 95, heating apparatus 100, switch 102, and back to the source of power. A cooling anticipation heater 101 is connected in parallel with switch 5361. For the operation of heating or cooling, the appropriate switch 101 or 102 is closed. Assuming switch 102 is closed, since magnet 50, as shown in FIGURE 2 is in the left-hand position, switch 5360 is closed and the heating apparatus is in operation. As the temperature of space and bimetal 13 increase, the bimetal would build up a force tending to move magnet 34 to the left away from stop 31. Before the magnet 34 can move away from stop 31, the force developed by bimetal 13 must be greater than the sum of a number of opposing forces. A first force to be overcome is the attractive force between magnet 34 and stop 31. This force is adjustable by the position of stop 31, and'the amount of ferromagnetic material in the magnetic field of magnet 34. The force can be lowered by raising the screw 31 to place less ferromagnetic material or steel in the flux circuit as shown in FIGURE 6. The control of this force by the position of the screw 31 is aided by positioning the screw closer to one face or pole of themagnet as shown in FIGURES 3 to 6. Another force to be overcome before the drive magnet 34 can move away from stop 31 is the lateral force developed by the repelling action between magnets 34 and 50. The lateral force is the force in the plane parallel to the front of housing 24. The lateral force depends on the distance between the magnets and the temperature of the magnets. There is a repelling force between magnets 34 and 50 which is perpendicular to the drawing of FIGURE 1; however, the perpendicular force only maintains a tension on blade 41 Two forces aid the bimetal in moving the magnet 34 away from the stop: (1) the lateral force developed by blade 40, (2) the spring tension of the bimetal itself. These forces are stored up in blade 40 andslbimetal 13 when magnet 34 snaps against stop 30 or When the force developed by bimetal 13 is sufficient to overcome the attractive force between magnet 34 and stop 31 and lateral force between magnets 50 and 34, magnet 34 moves from the left as shown in FIGURE 1 and FIGURE 4. As soon as the magnet moves away from the stop, the attractive force between the stop and the magnet sharply decreases and more power is available from the bimetal to overcome the lateral repelling force between the magnets. By properly designing the spring rate of the bimetal 13, magnet 34 will move away from stop 31 and against stop 30 with a snap action. A

number of force changes take place as the magnet moves from stop 31 to stop 30. When magnet 34 has moved to the'left past the center of magnet 50, the lateral repellingforce between the two magnets reverses in direction. The reversed lateral force drives switch magnet 50 to the right from the position as shown in FIGURES 2 and 4 to the position as shown in FIGURE 5.

,As magnet 34 moves in the vicinity of stop 30, the attractive force between magnet-34 and stop 30 acts on magnet 34 to pull it against the stop. The combined force resulting from the lateral repelling force between the two magnets and the attractive force between magnet 34 and stop 30 actually pulls magnet 34 against stop 30 even though bimetal 13 is acting on magnet 34 in the opposite direction. In other words, a force opposite to the force required to drive magnet 34 against stop 30 is stored up in bimetal 13. A similar force is stored up in blade 40.

By the proper positioning of stops 30 and 31 associated with drive magnet 34 and stops 70 and 71 associated with switch magnet 50, the force balance of the switch mech anism provides a desirable operational characteristic. First of all, the snap action provided upon the movement of drive magnet 34 from one stop to the other practically eliminates contact fade-off. In switch mechanisms, the current rating of the switch often depends upon the contact force between the movable and fixed contacts of the switch. Where the contact force decreases to zero before the switch opens, the resistance of the contact increases to form switch heat and a burning action which decreases the life of the switch. tween the stops slowly, the lateralrepeliing action between magnets 34 and '50 would drop to zero when the magnets reached an aligned position. When the lateral repelling force between the two magnets was zero, the contact force developed between framework 53 and blade 60 would be substantially zero. Such a condition would reduce the life of'the switch mechanism 12. By the use of the ferromagnetic stops, contact fade-off is overcome.

Another feature brought about by the proper selection of the stops previously mentioned is a reduction in the differential of the switch mechanism. When drive magnet 34 moves against eitherstop 30 or stop 31, it does so as a result of lateral force supplied by magnet 50 and the force of attraction between it and either stop. These forces are stored up in bimetal 13 and blade 40. Asa result of this stored up force, the temperature change of bimetal 13 in the opposite direction to produce movement of magnet 34 in the opposite direction is reduced. By the adjustment of ferromagnetic stops 30 and 31 to change the amount of flux flow 80 as shown in FIGURE 6, the attractive force between the magnet 34 and the stops is changed. The adjustment of the stops iseffective in changing the differential of the switch; that is, the temperature range which bimetal 13 must experience to obtain operation of switch mechanism 12.

Another feature provided by stops 30 and 31 is the contact pressure maintained between the movable contact 53 and the contact blades60 and 61. Referring to FIGURE 4, the lateral repelling force between magnets 34 and 50 provides a force to hold framework 53 against blade 60. If magnet 34 did not engage stop 30 but was free to move farther to the right, the lateral force between magnets 34 and 50 would be reduced and the contact pressure between contacts 53 and 60 would be lower. 7

Since the switch mechanism 12 can be used with a temperature control as shown in FIGURE 1, the effect of changes in ambient temperature must be considered. The magnets 34 and 50 have a high temperature coefficient of magnetization. Such a characteristic results in a change in the attractive force available between magnet 34 and the ferromagnetic stops 30 and 31 as the tempera ture changes. This results in a definite change in the force required to operate the switch or a change in the temperature differential of a thermostat. By selecting a If magnet 34 moved be- 6 temperature affected ferromagnetic material for the framework 35, a magnetic shunt is provided as shown in FIG- URE 6. Since the flux flow in the path affects the attractive force betweenmagnet 30 and stop 31, and the force remains substantially constant even though the temperature of the magnet 34 varies, the temperature change of bimetal 13 necessary to operate the switch remains constant. This feature is quite important as without the ferromagnetic shunt 35, the thermostat would have a changing differential which can adversely affect the cycling pattern of the control system. As the temperature of the magnet 34 increased, the temperature range which bimetal 13 would have to experience to operate the switch mechanism 12 would decrease.

The effect of switch heat on the control point of a heating control thermostat has the same effect as an anticipation heater. In fact, thermostats are presently available which use the switch heat for anticipation purposes. The use of the switch heat or heat from an anticipation heater effectively false-heats thethermostat to anticipate a temperature satisfaction of the thermostat to prevent an overshooting of the control point. 'When switch 5360 closes to energize heating apparatus, heater 94 provides heat anticipation. The opposite type of anticipation is quite common in cooling thermostats. When cooling is desired and switch 101 is closed, heater 101 is energized when switch 53-61 controlling cooling apparatus 93 is open. The heater heats the temperature rmponsive or bimetal 13 to increase the temperature in the thermostat above the spacetemperature to bring on the cooling equipment sooner than would be the case otherwise.

In a thermostat used for controlling cooling equipment,

heat developed by a switch has an adverse effect as it is in direct conflict with'the heat developed by the anticipation heater which is energized when the cooling control thermostat switch is open. Since magnet 50 is enclosed by framefork 53, the temperature of the magnet will increase when current is conducted through the framework. Let us assume that cooling equipment is connected to be energized when the circuit between terminal'strip 62 and terminal 26 is closed in a position when magnet 50 moves to the right as shown in FIGURE 5. The repelling force between magnets 34 and 50 decreases as the temperature of magnet 5t) increases. A reduction in the lateral repelling force between the magnets is the same as increasing the distance between the magnets which reduces the dif ferential of the switch mechanism. Since cooling an ticipation in effect reduced the differential of the thermostat, the effect of the switch heat on the magnet and the lateral force developed between the two magnets'overcomes the adverse effect of the switch heat. As a result, when the thermostat isconnected for controlling cooling equipment the switch differential remains substantially constant even though the total switch heat developed might increase as the on cycles become longer.

While various features of the present invention are disclosed in some detail in the present specification, the scope of the'invention is intended to be limited only by the appended claims.

Iclairn: I

1.An improved switching apparatus comprising, a switch permanent magnet means adapted to operate a switch, a resilient drive means, a drive permanent magnet means spaced adjacent said switch magnet means, means connecting said resilient drive means to said drive magnet means so upon movement of said drive magnet means by said resilient drive means said switch magnet means is moved, said drive magnet having a flux'density which varies with temperature and beingmounted for movement between a first and asecond ferromagnetic stop, said drive magnet means having a ferromagnetic shunt associated therewith, said shunt having a perme stops remains constant with changes in temperature.

2. In a thermostat, a base member, temperature responsive means mounted on said base member, a drive magnet, first means for supporting said drive magnet by said temperature responsive means so said magnet can move in a first plane with respect to said base, said temperature responsive means providing a resilient drive, said first means comprising a ferromagnetic material which has a high temperature .coefficient of permeability, said drive magnet having a high temperature coefficient of magnetization whereby upon an increase in temperature the magnetic fiux is reduced, at least one ferromagnetic stop positioned on one side of said drive magnet, a switch magnet pivotally supported for movement in a second plane parallel to said first plane, said drive and switch magnets repelling against each other by an amount depending upon the distance between said magnets, said switch magnet being adapted to close an electrical circuit in one extreme position, said drive magnet being attracted to said stop by a force which is constant with temperature since the effective magnetic flux flow between said drive magnet and said stop is contsant.

3. In an improved switch, a switch magnet adapted to move in a first plane between two positions to control associated electrical circuits, a drive magnet pivotally supported for resilient movement adjacent said switch magnet for movement in a second plane parallel said first plane, said drive magnet having a flux density which varies with temperature, a first and a second ferromagnetic stop spaced in said second plane to limit the movement of said drive magnet, said magnets repelling each other so said switch magnet moves to one of said positions and said drive magnet moves in an opposite direction against one of said stops, ferromagnetic shunt associated with said drive magnet, said shunt having a magnetic permeability which changes With temperature to compensate for the efiect of temperature on said flux density.

4. In an improved thermostat, a switch magnet mounted for movement between a first and second position to operate a switch, a'drive magnet supported for movement adjacent said switch magnet, said magnets being oriented to repel each other, temperature responsive means, resilient means connecting said temperature responsive means to said drive magnet, at least one ferromagnetic stop, said temperature responsive means being adapted to move said drive magnet toward said one stop upon a predetermined change in temperature, said switch magnet is repelled to operate said switch, means supporting said drive magnet to maintain a constant attractive force between said drive magnet and said stop independent of temperature.

5. In a thermostat, a base member, temperature responsive means mounted on said base member, a first magnet, resilient means connecting said first magnet and said temperature responsive means to move said magnet in a first plane with respect to said base, adjustable connection means flexiblyconnecting said magnet to said base to determine the distance between said base and said first plane, first and second ferromagnetic stops spaced on opposite sides of said first magnet and'adjustable to select the amount of ferromagnetic material in the magnetic circuit when said magnet is against one of said stops, said stops being spacedat a distance greater than the width of said magnet so said magnet can be moved between said stops, a housing attached to said base, a second magnet contained in said housing, said second magnet being pivotally supported for movement in a second plane parallel to said first plane, said first and second magnets having their like poles adjacent to each other so said magnets repel against each other, said second magnet carrying at least one electrical contact and having an electrical connection to a first terminal outside said housing, a second contact inside said housing cooperating with said one contact when said magnet is moved to one side of said housing, a second electrical terminal outside said housing connected to said second contact, said first and second magnets repelling each other so that when said first magnet is against said first stop,

8, said second magnet is repelled in one direction and when said first magnet is moved by said responsive means toward said second stop said repelling force causes said second magnet to move in a direction opposite to said one direction and when said first magnet is moved against said second stop, said second magnet effectively closes said circuit when in said last mentioned position, means for connecting said terminals in a control circuit, and a ferromagnetic shunt mounted on said first magnet, said shunt having a magnetic permeability which changes with temperature so the force between said first and second magnets is independent of the temperature of said magnets.

6. In a thermostat, a base member, temperature responsive means having a resilient output means, a first magnet, means connecting said first magnet and said output means of the temperature responsive means to move said magnet in a first plane with respect to said base, adjustable connection means flexibly connecting said magnet to said base to maintain the distance between said base and said first plane constant, first and second adjustable ferromagnetic stops spaced on opposite sides of said first magnet to select the amount of ferromagnetic material in the magnetic circuit when said magnet is against one of said stops, said stops being spaced at a distance greater than the width of said magnet so said magnet can be moved between said stops, a housing attached to said base, a sec ond magnet contained in said housing, said second magnet being pivotally supported for free movement in a second plane parallel to said first plane, said first and second magnets having their like poles adjacent to each other, said second magnet carrying at least one electrical contact and having an electrical connection to a first terminal outside said housing, a second contact inside said housing cooperating with said one contact when said magnet is moved to one side of said housing, a second electrical terminal outside said housing connected to said second contact, said first and second magnets repelling each other so that when said first magnet is against said first stop, said second magnet is repelled in one direction and when said first magnet is moved by said responsive means towards said second stop said repelling force causes said second magnet to move in a direction opposite to said one direction and said first magnet is moved against said second stop against said resilient output means, said second magnet effectively closes said circuit when in said last mentioned position, and means for connecting said terminals in a control circuit.

7. In a thermostat, a base member, temperature responsive means mounted on said base member, a first magnet, resilient means connecting said first magnet and said temperature responsive means to move said magnet in a first plane with respect to said base, first and second ferromagnetic stops spaced on opposite sides of said first magnet, said stops being spaced at a distance greater than the width of said magnet so said magnet can be moved between said stops as the temperature of said responsive means varies through a predetermined temperature range, a second magnet, said second magnet being pivotally supported for movement in a second plane parallel to said first plane, said first and second magnets having their like poles adjacent to each other so said magnets repel against each other, said second magnet carrying at least one electrical contact and having an electrical connection, a second contact cooperating with said one contact when said magnet is moved to one side, said first and second magnets repelling each other so that when said first magnet is against said first stop, said second magnet is repelled in one direction and when said first magnet is moved by said responsive means toward said second stop said repelling force causes said second magnet to move in a direction opposite to said one direction and when said first magnet is moved against said second stop, said second magnet effectively closes said circuit when in said last mentioned position, means for connecting said 9 contacts in a control circuit, and means for heating said second magnet proportionally to said current flow in said circuit, said second magnet having a permeability which varies with temperature so that as heat is developed by said switch to effect said range said repelling action. is modified to compensate for the adverse effects of said switch heat.

8. In athermostat, a base member, tempenature responsive means mounted on said base member and adapted to have a resilient output in response to a space temperature, a first magnet connected to said temperature responsive means and adapted to move in a first plane with respect to said base, first and second ferromagnetic stops attached to said base member and spaced on opposite sides of said first magnet, said stops being spaced at a distance greater than the dimension of said magnet as in a direction between said stops, a second magnet pivotally supported for movement in a second plane parallel to said first plane whereby magnetic coupling exists between said magnets, a switch associated with said second magnet, said second magnet being supported by a current carrying member forminga part of said switch so said second magnet is heated when said switch is closed, said switch being closed to energize temperature conditioning apparatus when the temperature of said space responsive means deviates from a predetermined value,

said coupling being a repelling force so that when said first magnet is against said first stop said second magnet is repelled in the opposite direction to actuate said switch, said repelling force varying with the temperature of said second magnet, a heater associated with said temperature responsive means to artificially heat said responsive means, said heater having the opposite effect as heat developed by said switch on said responsive means, however the change in said repelling force as said second magnet is heated by switch heat compensates for the adverse eifect of said switch heat on said temperature responsive means.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN IMPROVED SWITCHING APPARATUS COMPRISING, A SWITCH PERMANENT MAGNET MEANS ADAPTED TO OPERATE A SWITCH, A RESILIENT DRIVE MEANS, A DRIVE PERMANENT MAGNET MEANS SPACED ADJACENT SAID SWITCH MAGNET MEANS, MEANS CONNECTING SAID RESILIENT DRIVE MEANS TO SAID DRIVE MAGNET MEANS SO UPON MOVEMENT OF SAID DRIVE MAGNET MEANS BY SAID RESILIENT DRIVE MEANS SAID SWITCH MAGNET MEANS IS MOVED, SAID DRIVE MAGNET HAVING A FLUX DENSITY WHICH VARIES WITH TEMPERATURE AND BEING MOUNTED FOR MOVEMENT BETWEEN A FIRST AND A SECOND FERROMAGNETIC STOP, SAID DRIVE MAGNET MEANS HAVING A FERROMAGNETIC SHUNT ASSOCIATED THEREWITH, SAID SHUNT HAVING A PERMEABILITY VARYING WITH TEMPERATURE SO THAT THE FORCE DEVELOPED BETWEEN SAID DRIVE MAGNET MEANS AND ONE OF SAID STOPS REMAIN CONSTANT WITH CHANGES IN TEMPERATURE. 